1. Field of the Invention
The present invention relates to a valve timing control apparatus for an engine, and more particularly to a valve timing control apparatus for an engine to realize a stable engine revolution irrespective of fuel type when the engine is cold.
2. Description of the Related Art
In general, the opening/closing timing of intake valves and exhaust valves has a great influence on engine power, and there may be a case where a valve overlap range is provided by changing settings as to the opening/closing timing. In the valve overlap range, the valve overlap amount, which is set such that the operating angles of the intake valves and the exhaust valves are overlapped, improves intake efficiency because exhaust gas can be pushed out toward an exhaust passage due to inertia of intake air when the engine is revolving at a high speed.
The valve overlap range needs to be changed according to requirements as the engine speed is changed from a low speed to a high speed. Specifically, when a MPI (multi-port injection) type engine, in which fuel is injected via an intake port in the exhaust stroke of a piston, is used, fuel adheres to the wall of the intake port resulting in tardy acceleration at cold-start of the engine, for example. Therefore, the valve overlap range needs to be changed according to the operative condition of the engine. Accordingly, a valve timing control apparatus has been disclosed which changes the overlap where both intake valves and exhaust valves are opened according to the operative condition of an engine (Japanese Laid-Open Patent Publication (Kokai) No. 10-176557, for example)
The valve timing control device reduces internal EGR by reducing the overlap amount when the engine is cold. This prevents tardy acceleration, and reduces internal EGR, thus ensuring combustion stability.
Further, the valve timing control device determines carburetion characteristics of fuel being used for the engine. When it is determined that light fuel with favorable carburetion characteristics is being used, the overlap amount is increased to be larger than in the case where heavy fuel with unfavorable carburetion characteristics is used. Therefore, even when the engine is cold, fuel is unlikely to adhere to the wall of an intake port or the like depending on the fuel type.
Further, if combustion is unstable due to a variation in air-fuel ratio, e.g., at a cold-start of the engine, the overlap amount is reduced close to zero such that the closing timing of the exhaust valves and the opening timing of the intake valves correspond to each other at an exhaust top dead center (TDC) of a piston. This reduces internal EGR to ensure combustion stability.
By the way, reducing the overlap amount close to zero at a cold-start of the engine reduces internal EGR as described above, and hence combustion stability can be ensured. In this case, however, the amount of exhaust gas blown back toward the intake port is decreased, and the percentage of exhaust gas which has been emitted once and taken again into cylinders is decreased and may cause some problems. Specifically, reducing internal EGR raises the problem that fuel carburetion is deteriorated since fuel atomization cannot be accelerated when the engine is cold-started. Particularly in the case where heavy fuel with unfavorable carburetion characteristics is used, the air-fuel ratio becomes excessively lean, causing unstable engine revolution and engine stall.
To address this problem, it can be envisaged that the air-fuel ratio is made richer by increasing the amount of fuel, but this raises the problem that unburned fuel (HC) is increased at the cold-start of the engine and deteriorates the exhaust gas performance.
Therefore, it is necessary to take some measures against unstable engine revolution when the engine is cold-started. According to the above described prior art, however, although the overlap amount is set to be small when the engine is cold, this setting of the overlap amount is made after carburetion characteristics of fuel are determined. Specifically, although the overlap amount can be changed according to carburetion characteristics of fuel, special consideration is not given to the overlap amount when the engine is cold-started, causing unstable engine revolution and engine stall depending on the fuel type.